Chlorophyll fluorescence as measured in situ by animal-borne instruments in the northeastern Pacific Ocean

Chlorophyll concentration in the ocean is a metric for phytoplankton biomass, which forms the base of most pelagic food webs and is a critical component of the planet's carbon cycle. Phytoplankton biomass has been designated an Essential Ocean Variable (EOV), but in situ chlorophyll measurements are challenging and expensive to obtain, especially in remote regions. We deployed 11 Conductivity-Temperature-Depth-Fluorescence (CTDF) instruments on 15 northern elephant seals (Mirounga angustirostris) during 2014, 2018, and 2019 collecting 2532 temperature, salinity, and chlorophyll fluorescence profiles that exceeded 180 m depth and extended over 2000 km from the nearest landmass. The tags' fluorometers were calibrated in a laboratory and a controlled field setting approximating northeastern Pacific phytoplankton assemblages and conditions, resulting in adjustments to manufacturer calibration curves ranging from -4% to 81%. Subsurface chlorophyll maxima were observed below the first optical depth in 69.2% of the casts, of which 24.9% were deep chlorophyll maxima below the mixed layer. Established fluorescence quenching correction methods were not applicable to this dataset; however, time of day did not introduce a consistent bias to our results. In situ chlorophyll measurements were on average 5.24 times higher than coincident satellite measurements. This in situ chlorophyll dataset measures an EOV in high resolution in remote regions at low cost, a valuable contribution to the Global Ocean Observing System (GOOS). Regional index terms: North Pacific Ocean (35-50 degrees N, 120-180 degrees W].